In heart muscle, as in other tissues required to sustain continued high levels of work output, the energy transduction processes occurring in the inner mitochondrial membrane are vital to tissue health. Cytochrome c, a protein component of the mitochondrial terminal respiratory chain which is readily obtainable from heart tissue in pure form, plays a central role in such processes. Even though a wealth of information is available concerning its reaction kinetics with other respiratory chain components, its amino acid sequence and its spatial structure, effective protein molecular details of overall functional activities are lacking, precluding an understanding of mechanisms and thereby precluding any possibility of their control. Following the purification of a large number of chemical derivatives of the protein, it has become possible to map precisely the areas on the surface of cytochrome c which bind its various physiological electron exchange partners and the various anions which specifically affect the reaction with these enzyme systems. This development, together with the related knowledge of the kinetics of the enzyme's reactions, have opened the way to an examination of how the parameters known to affect cytochrome c activity in vitro operate under in vivo conditions in intact mitochondria. This investigation program is expected to lead to an understanding of the physiological regulation of cytochrome c function.